In recent years, a considerable number of treatment systems have been developed for the purification of residual waters. Most of these systems require sophisticated and costly equipment to guarantee the satisfactory operation of the procedure.
Also, the impact of organic wastes dumped into a water stream can be measured through the dissolved oxygen, along with the nitrogen-ammonium, chemical oxygen demand (COD) and biological oxygen demand (BOD). In the aerobic treatment of residual waters, i.e., the treatment in which there is a strong increase in oxygen supply via irrigation of solid surfaces, by agitation or simultaneous agitation and aeration, to assure that growth of the microorganisms that will degrade the organic matter contained in the waste water and their activity grow proportionally to the rate of aeration. Aerobic digestion is a bacterial process in which bacteria consume organic matter and convert it to carbon dioxide in the presence of oxygen. Once there is a lack of organic matter, the bacteria die and are used as aliment by other bacteria; the reduction of solids also occurs in this phase and because aerobic digestion happens much faster, the capital costs of this process can be much lower depending on the scale of the project.
However, the exploitation costs are much higher for aerobic digestion given the energetic costs necessary to add oxygen to the process.
Conversely, anaerobic treatment is a bacterial process carried out in the absence of oxygen. Anaerobic digestion generates biogas, which contains a high proportion of methane that can be used fuel. Generating this fuel is a dominating advantage for this anaerobic process. Its disadvantage is the time needed for the process, which in some instances may take several days, but nevertheless, the capital costs may be than those for the aerobic treatment.
When residual waters with a reasonably high organic load are kept in the pond for several days, anaerobic sediment is accumulated at the base of the pond. In an uncovered pond, the activity of anaerobic digestion is conducted at the base or bottom of the pond, while the activity near the surface tends to be aerobic facultative. These ponds may be shut to air with a floating cover to improve the activity of anaerobic digestion through excluding air, permitting the collection of biogas fuel, and reducing the effect of the odor coming from the anaerobic activity. Generally, these ponds may take residual waters from BOD5 values between 400 and 5000 g/m3 with time of retention between 4 and 7 days. The anaerobic process is mostly self-propelled and the only initial mechanical action required is that of supplying the pond with residual waters and forcing its exit toward a drain by overflow.
The U.S. Pat. No. 4,209,388 document describes a method for residual water treatment that includes a first stage in which the body of residual water is introduced into a reservoir outfitted with an air supply that contains algae for the decomposition of organic matter, then the residual water is transferred to a second reservoir where there are no nutrients or solar light, as a consequence of the latter the algae die and sediment. The residual water is thereafter transferred to a third reservoir to separate the water from the sedimented algae.
The U.S. Pat. No. 4,267,038 document presents a system of residual water purification, which includes the step of elimination of solids like sludge, the digestion of said sludge, and the subsequent remixing with residual water, then an anaerobic stage is applied where bacterial oxidation of organic wastes takes place. The next steps include the stabilization of nutrients, nitrification, de-nitrification, and re-aeration, followed by the transference of the residual waters from the anaerobic tanks to one or more tanks containing algae or aerobic bacteria. The treated residual water is sent to multiple tanks including the recycling of the flow current to the aerobic or anaerobic tanks indicated previously, concomitantly with the separation of the algae used.
U.S. Pat. No. 5,447,850 presents a method to obtain methane from the treatment of residual waters. The method includes the use of aerobic and anaerobic microorganisms, which are inoculated in the residual water, the flow current is fermented with said organisms and the methane produced is conveniently removed.
U.S. Pat. No. 5,744,041 reveals a method for reducing BOD5 in waste material that includes a high concentration of organic wastes. The method includes the steps to obtain two fractions of the residual water in a liquid fraction that includes water and waste organic matter, thereafter a portion of said waste organic matter anaerobically digested by microorganisms present in the wastes. Then comes the removal of a portion of the liquid fraction that has a reduced BOD5 in relation to the BOD5 from waste material; said portion that has been removed is then mixed with aerobic microorganisms and air is injected. A portion of the waste material that has been digested by aerobic microorganisms is allowed to form liquor that includes water and suspended solids. Afterwards, a portion of the suspended solids is sedimented, permitting the formation of clarified liquor with a reduced BOD5 with respect to the BOD5 of the liquor. The clarified liquor is subjected to bacterial treatment to obtain a permeate with a reduced BOD5 with respect to the clarified liquid. As a final stage, at least a portion of the permeate is discharged or reused.
In spite of the existence of the previously discussed, there is still a need to furnish a reactor and a process to carry out an anaerobic treatment in residual waters, which provides an efficient solution in aspects related to the optimization of mix processes between the biomass and the substrate, the devices employed for the retention of the biomass, and the separation and recovery of such. With the solution proposed by the current invention, the conversion is achieved from a low-rate traditional system like the conventional anaerobic pond to a more compact and efficient system that additionally permits recovery of clean energy in the form of biogas. An important characteristic maintained in the high-rate bioreactor, HRAPB, is its simple operation and maintenance, which are reflected in low operational costs, making an HRAPB unit sustainable in the vast majority of minor municipalities and small communities in rural sectors throughout the world.